Excessive entry of Ca ions into neurons not only disrupts cellular function but will initiate autodestructive pathological processes that seem designed to guarantee the demise of the cells. Based on the observation that extracellular Ca ionic activity ([Ca+2]e) falls to low levels in injured spinal cords, much lower than expected from equilibration of intracellular and extracellular fluids, we hypothesize that phosphates released from lipid peroxidation bind Ca ions and cause the profound depression of [Ca+2]e. We further hypothesize that the [Ca+2]e depression protects cells that survive the initial trauma but that cells load up with Na ions during this period. Finally ,we propose that [Ca+2]e recovery causes secondary tissue damage, related to excessive Ca/Na exchange across cell membranes. A similar type of injury has been described in cardiac tissues perfused with Ca-free solutions and reperfused with normal Ca-containing solutions. This phenomenon called Ca paradox may explain several hitherto puzzling characteristics of the acute response of the spinal cord to trauma. We will test these three hypotheses in a graded weight drop contusion model of spinal cord injury in rats. Tissue Na, K, Ca, and phosphate changes will be correlated at different times after graded contusion injured. Specific predictions of each hypothesis will be tested. Soluble, bound, and total tissue phosphates will be related to tissue and extracellular Ca changes to determine whether phosphates are released, bind Ca, and cause the [Ca+2]e depression. We will assess the effects of U74006F (a 21-aminosteroid lipid peroxidation inhibitor) on the relationships of Na, Ca, and phosphate changes. We will manipulate [Ca+2]e at the lesion site (monitored with ion-selective microelectrodes) to see if rapid [Ca+2]e normalization causes more tissue damage. Treatments that may reduce Na+/Ca+2 exchange will be assessed, i.e. hypothermia, magnesium, and potassium. We will measure ionic shifts in injured spinal cords maintained at 37oC, 27oC, and 17oC to ascertain whether cooling produces expected changes in tissue [Na]:[Ca] ratios and phosphates at different times after injury. Solutions containing high concentrations of Mg+2 and K+ will be applied to the spinal cord to see if raising [Mg+2]e and [K+]e levels will reduce Na+/Ca+2 exchange. If successful, these experiments may explain the presence of a therapeutic window in the immediate post-traumatic period and provide a rational basis of developing treatments for acute spinal cord injury.